The music files are coded in whatever programming language format and thus any new music files would have to be coded in that format. Secondly you would have to find what code controls the sims animations of playing the piano or guitar and copy those into the commands to control the animation with the music. Then you have to also make sure that the command wheel that you have to select the song from the various commands contain the new songs that you have placed into the command queue. And unfortunately, I don't think anybody has gone that far to mess with the coding that directly affects the game. At least I haven't found a mod that adds more music to the "instruments" played.
